A combination of palbociclib and cetuximab demonstrated substantial antitumor activity among patients with platinum- or cetuximab-resistant HPV-unrelated head and neck squamous cell carcinoma, according to results of a multigroup phase 2 trial published in The Lancet Oncology.

“Currently, effective therapeutic options for patients with cetuximab-resistant HNSCC are few. Traditional chemotherapy has marginal activity, with 6% of patients or fewer achieving a tumor response,” Douglas R. Adkins, MD, professor in the oncology division of the department of medicine at Washington University School of Medicine in St. Louis, and colleagues wrote. “The most effective therapy for these patients might be pembrolizumab [Keytruda, Merck] or nivolumab [Opdivo, Bristol-Myers Squibb], which have resulted in responses in 11% to 16% of patients and median OS of 6.9 months to 8 months. Novel treatment strategies are needed for patients with recurrent or metastatic HNSCC.”

The combination of the cyclin-dependent kinase (CDK) 4/6 inhibitor palbociclib (Ibrance, Pfizer) and epidermal growth factor receptor inhibitor cetuximab (Erbitux, Eli Lilly) appeared safe and tolerable in the phase 1 portion of the multicenter trial, conducted across eight U.S. university sites.

For phase 2, Adkins and colleagues divided 62 patients with HPV-unrelated HNSCC (median age, 66 years; interquartile range [IQR], 58-70; 71% men) into two groups: those who were platinum-resistant (group 1; n = 30) and those who were resistant to cetuximab (group 2; n = 32). Primary tumor sites included the oral cavity (42%) and larynx (29%), and 81% of patients had received one or two prior lines of treatment for metastatic or recurrent disease.

All participants received oral palbociclib (125 mg daily on days 1-21) and IV cetuximab (400 mg/m2 on day 1 of cycle one, followed by 250 mg/m2 once weekly) in 28-day cycles. Objective response, defined as complete and partial responses per RECIST 1.1 criteria, served as the primary endpoint.

Researchers followed patients in group 1 for a median 5.4 months (IQR, 4.4-12.1) and those in group 2 for a median 5.5 months (IQR, 4.3-8.3).

In each group, only one patient with a tumor response previously had received immunotherapy.

The most prevalent grade 3 to grade 4 adverse event associated with palbociclib was neutropenia, which occurred in 34% (n = 21) of all patients. The researchers did not document any treatment-related deaths.

The researchers cited various limitations to their study, including its single-group design, and noted that the results will need to be confirmed in a controlled trial with a larger sample size. They acknowledged that immunotherapy might have affected OS outcomes, and that the study design did not permit the evaluation of whether palbocilib’s antitumor activity occurred directly or by reversal of primary cetuximab resistance.

These data suggest a need for further study of palbociclib in patients with recurring or metastatic HNSCC, according to a related editorial by Garth W. Strohbehn, MD, hematology/oncology fellow at University of Chicago, and Everett E. Vokes, MD, professor of medicine and radiation oncology physician-in-chief at University of Chicago Medicine.

“However, we should be circumspect about the prospect of CDK 4/6 inhibitors as standardized, cost-effective therapies in recurrent and metastatic HNSCC,” the authors wrote. “Bringing this class of drugs to head and neck oncology clinics, as either monotherapies or immunotherapy partners, will require appropriately controlled studies linked to biomarker evaluation with both survival and cost-effectiveness endpoints.” – by Jennifer Byrne